Tumor promoters are those compositions that effect the formation of tumors in those cells that have been previously initiated. Initiation is an event which appears to damage the DNA of cells, but does not necessarily result in tumor formation. Exposure of initiated cells to promoting agents leads to carcinomas and papillomas. Prior to the present invention, there has been no simple, quick and inexpensive test for determining whether a composition is a tumor promoter or has tumor promoting activity. One existing test involves painting mouse skin with suspected compositions. Demonstration of promoting activity requires repeated application of the composition over several months before carcinomas and papillomas are detected. This analysis is expensive and time consuming.
A second category of compositions are known as complete carcinogens. These are capable of inducing tumors by themselves without the prior requirement of exposure to initiators or treatment with the promoters. Such compositions appear to combine both initiating and promoting activity. Prior to the present invention, compositions to which humans are exposed, such as food additives, topical products, or the like, must be routinely tested to determine their activity for causing cancer in humans. Long term tests for carcinogenic compositions expose animals to suspected compositions and measure tumor formation. Such tests are labor intensive, expensive and produce variable results. A widely used short-term test, and one that does not require animal testing, is known as the Ames test. This test is of limited utility and can only determine whether a composition damages DNA sufficiently to result in mutagenesis. The Ames test cannot measure tumor-promoting activity of compositions nor detect that activity of a carcinogen which is not mutagenic. The Ames test involves the activation of a composition using a microsomal extract from rat liver. The activated composition is mixed with a bacterial strain bearing a mutation. Mutagenic compounds are detected by their ability to reverse the mutation, thereby allowing the bacteria to grow in a restrictive medium. Thus, carcinogens are identified on the basis of their ability to induce specific mutations.
While most complete carcinogens cause mutagenesis, not all mutagens are complete carcinogens. Furthermore, the Ames test is not capable of determining whether a composition is a tumor promoter. For example, the Ames test is not capable of determining which components of tobacco smoke are tumor promoters nor can it detect the tumor promoting activity of dioxin (TCDD). Thus, the Ames test is seriously deficient, since both dioxin and tobacco smoke are well known tumor promoting agents common in the environment.
Thus, the need persists for an accurate, inexpensive and definitive short-term test for determining whether a particular composition is a tumor promoter or has tumor promoting activity. Such a test should also establish whether a known mutagen has promoter activity and therefore is apt to behave as a complete carcinogen. Furthermore, it would be desirable to provide a test which is capable of making such a determination directly at extremely low dilutions in the order of one part per billion or less.
If a simple and inexpensive, yet highly accurate assay could be provided to identify the presence of tumor promoters, an important tool in the prophylaxis and management of cancer would be provided.
One such prior art attempt at such an assay was developed by Penman et al and is described in U.S. Pat. No. 4,569,916 which issued Feb. 11, 1986. While representing a major step forward in the context of the times, the assay described therein suffered from the fact that it was dependent upon $20,000 to $30,000 of equipment and required several hours to complete.
Accordingly, a need still exists for the development of an accurate, quick and inexpensive assay for the detection of tumor promoters and tumor promoting activity and it is toward this goal that the present invention is directed.